When conducting electron microscopy (EM) analysis, there are a few important parameters that must be taken into account to produce the best possible results, and to image the feature of interest. One of the crucial roles is played by the voltage (or tension) applied to the source electrodes to generate the electron beam. Historically, the trend has always been to increase the voltage to improve the resolution of the system.
It is only in recent years that scanning electron microscope (SEM) producers have started to focus on improving the resolution at lower voltages. A major role in this has been the expanding field of application of EM to the life sciences - especially after the introduction of the Nobel prize-winning cryo-SEM technique. This blog will focus on the effects of the voltage on the results of electron microscopy analysis.
The electron beam voltage shapes the interaction volume
The voltage is an indication of the electrons’ energy content: this will therefore determine what kind of interaction the beam will have with the sample. As a general guideline, a high voltage corresponds with a higher penetration beneath the surface of the sample —also known as bigger interaction volume.
This means that the electrons will have a larger and deeper propagation within the sample and generate signals in different parts of the affected volume. The chemical composition of the sample also has an impact on the size of the interaction volume: light elements have fewer shells, and the electrons’ energy content is lower. This limits the interactions with the electrons from the electron beam, which can therefore penetrate deeper into the sample, compared to a heavier element.
When analyzing the outcoming signals, different results can be obtained. In desktop instruments, three kinds of signals are normally detected: backscattered electrons (BSE), secondary electrons (SE), and X-rays. As digging into the different nature of the signal is not the main focus of this article, more info can be found in this blog.
The effect of voltage in SEM imaging
The effect of voltage within the BSE and SE imaging is comparable: low voltages enable the surface of the sample to be imaged; high voltages provide more information on the layer beneath the surface. This can be visualized in the images below, where low voltages make surface sample contamination clearly distinguishable, while higher tensions reveal the structure of the surface underneath the contamination layer.
Figure 1: BSE images of tin balls at 5kV (left) and at 15kV (right). With the lower voltage, the carbon contamination on top of the sample becomes visible. When the voltage is increased, the deeper penetration enables the imaging of the tin ball surface underneath the carbon spots.
The nature of the sample is also hugely important in the choice of the appropriate voltage. Biological samples, several polymers, and many other (mostly organic) samples are extremely sensitive to the high energy content of the electrons. Such sensitivity is further enhanced by the fact that the SEM operates in vacuum. This is the leading reason why the focus of SEM developers is moving towards increasing the resolution value at lower voltages, providing important results even with the most delicate samples.
The main difficulty that is encountered in this process is the physics principle behind the imaging technique: in a similar way to photography, there are in fact several kinds of distortion and aberration that can affect the quality of the final output. With higher voltages, the chromatic aberrations become less relevant, which is the main reason why the previous trend with SEM was to turn towards the highest possible voltage to improve imaging resolution.
The generation of X-rays in a SEM
When it comes to X-ray generation, the story is totally different: a higher voltage is responsible for a higher production of X-rays. The X-rays can be captured and processed by an EDS (energy dispersive spectroscopy) detector to perform compositional analysis on the sample.
The technique consists of forcing the ejection of an electron in the target sample by means of the interaction with the electrons from the electron beam (primary electrons).
A charge vacancy (hole) can be generated in the inner shells of an atom, and it is filled by an electron with a higher energy content from an outer shell in the same atom. This process requires the electron to release part of its energy in form of an X-ray. The energy of the X-ray can finally be correlated to the atomic weight of the atom through the Moseley’s law, returning the composition of the sample.
The key factors in X-ray production are the following:
- Overvoltage: the ratio between the energy of the incoming beam and the energy necessary to ionize the targeted atom
- Interaction volume: defines the spatial resolution of the analysis
The ideal analysis requires a minimum overvoltage value of 1.5, which means that by increasing the electron beam voltage, the maximum number of detectable elements increases. On the other hand, a high voltage corresponds with higher chances of sample damage and, even more importantly, a larger interaction volume.
This does not only mean that the sample reliability could be compromised, but also that the generation of X-rays interests a much larger volume. In case of multilayers, particles, and generally non-isotropic materials, a larger interaction volume will generate signals coming from portions of the sample with a different composition, compromising the quality of the results.
Typical recommended tension values for the analysis range between 10 and 20kV, to balance the two effects. Choosing the ideal value depends on an additional aspect of EDS analysis that is known as ‘peak overlap’. X-rays generated by electrons moving from different shells of different elements can have comparable energy contents.
This requires more advanced integration processes to deconvolute the peaks and normalize the results, or use the higher energy content lines (coming from one of the two elements with overlapping peaks). While the former is already implemented in most EDS software, the latter is not always possible, considering that the higher energy level line for a very common element such as lead would require a voltage higher than 100kV.
More about EDS in SEM
If you would like to take an even deeper dive into EDS in scanning electron microscopy, take a look at this specification sheet for the Phenom ProX desktop SEM. Among other things, it demonstrates how you can control a fully-integrated EDS detector with a dedicated software package — and avoid the need to switch between external software packages or computers.
Check out the desktop SEM that generates the highest number of X-rays in its market segment — download the ProX specification sheet today:
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